Yay I was right about the scar is what let him know last time.
Though wrong about why there is a need for a double. So it was not a broken chastity, but a foul poison.
And one with a difficult antidote apparently. So at minimum they were able to keep her alive, but either took a large amount of time to procure an antidote.
Or took a long time to physically recover. Or both.
And in High Society with cousins like that, having the appearance of weakness is fatal, and can damage your reputation quite harshly.
More so when it becomes public that it was your own subsidiary that did it, but you cannot prove it.
And maybe the reason they left her in place till after the baby, is she became a Queen Mother.
Killing a queen with no child, one that isn't loved by the Emperor, is far less of a crime as killing a Queen Mother, and probably easier to do.
So even if the king didn't love her, he would protect them both with his power now as she has a his child now, most likely out of obligation.
Safer to have her act as a lamb in the open, and only replace her when you know she will be give a barn. (in this case it be given the Castle)